1. Field of the Invention
The present invention relates to a tendon (rod) gripping apparatus primarily for gripping smooth and/or deformed or multiple stranded tendons (rods), and more particularly, to an apparatus including a housing (barrel) for receiving a tendon (rod), a plurality of elongated jaw (wedge) members, a partially externally threaded cylindrical tube device to tighten and align the jaws (wedges) and to cause initial engagement of the jaws (wedges) to the tendon (rod) by wedging the jaws (wedges) within the housing (barrel) between the tendon (rod) and inner wall of the housing (barrel), a partially internally threaded tensioning device to properly position the apparatus.
As used herein, the term tendon is interchangeable with the term rod. Tendons (rods) are primarily a tensile unit when used in the Art of Construction, meaning that forces are induced normal to the longitudinal axis of the tendon (rod).
Tendons (rods) are utilized in the Art of Construction, in particular concrete construction in order to add to the strength of structures as is the case with concrete reinforcement (rebar and pre & post tensioning devices), and as is the case with soil or rock embankment stabilization devices (soil nails, rock anchors) to name a few that are most common.
Tendons are commonly used in a temporary load bearing but non-structural capacity (that which does not add strength to the completed structure). In one such instance, tendons are known to the art as formties. In vertical concrete construction, a cementitious material is placed between erected walls, termed formwork, which provide support until the concrete curing, hardening process is completed. Tremendous force is often exerted upon the formwork, particularly when large volumes of concrete are placed. Tie-rods, termed formties, are passed through holes drilled in the forms to prevent an outward expansion of the erected forms during placement and initial hydration, set, of concrete walls.
The formwork typically includes beams and planks or the like (“wales”, and “stiffbacks”, as known in the construction art). A tendon-gripping apparatus is used to prevent the formwork from sliding along the form tie. The formwork, in turn, serves as a guide for the formtie and as a platform against which the tendon-gripping apparatus is positioned. The force-distributing construction of the formwork supports the erected walls and prevents outward bulging of the walls while the fluid concrete is hardening.
Although concrete construction techniques have progressed tremendously over the last 50 years, most formties have not changed. The use of steel formties is particularly problematic because of the need to avoid rust which can destroy a structure or ruin its appearance. Prior attempts to eliminate structural rusting include either entirely removing the steel tendon or breaking the tendon back to a distance below the surface. The resultant hole is then plugged and patched over with cementitious material. These practices are very labor intensive and expensive. Unfortunately, the patching often results in an unprofessional finish, or worse, is overlooked, or simply fails to prevent rusting from occurring.
The use of deformed or multiple stranded tendons in the mode of concrete reinforcement (rebar) is within the field of knowledge of most laymen even though not intimately involved with the Art of Construction. In some cases this type of tendon is used as a formtie, as noted above, or in embankment stabilization, as noted below.
Soil or rock embankment stabilization is a particular construction art whereby an unstable elevation such as a hill, mountain, or cliff, having a substantially vertical face that is prone to catastrophic landslides is stabilized and rendered safe. External stabilization may be accomplished in a number of ways; by using netting made of various materials or stacking rock filled mesh baskets, know as gabions, at the face of the embankment to restrain the embankment. Internal stabilization is accomplished by drilling a hole in the embankment to a pre-determined depth and inserting a tendon of a pre-determined diameter somewhat less than that of the hole, to approximately the hole depth, and filling the annular space between the tendon and the pre-drilled hole with a cementitious mixture creating a bond between the tendon and the embankment. The tendon may be smooth, threaded or deformed such as re-bar or multiple stranded tendons and is secured to a retaining wall structure built at the face of the embankment that is either of concrete or steel. Threaded tendons may be secured via threaded apparatus such as common threaded nuts. Smooth or deformed cylindrical tendons, or multiple stranded tendons, may be secured via a device such as the instant invention.
The internal embankment forces that lead to embankment failure are transferred to the tendon and from the tendon to the retaining wall structure. The retaining wall structure captures any slough from the face of the embankment. In the case of a concrete retaining wall structure, formwork is constructed as noted above.
The tendon gripping apparatus disclosed herein is beneficially capable of gripping smooth and/or deformed, or multiple stranded fiberglass tendons (rods), thereby eliminating the problem of structural failure due to steel tendon corrosion. Furthermore, the tendon gripping apparatus, which includes a unique jaw assembly or jaw cluster, a means to set and align the jaws, and a tension device to properly place the apparatus against the formwork or embankment retaining.
The ultimate, failure strength of various tendons (rods) is established through very detailed laboratory test involving gripping devices that cannot be practically, or cost effectively used in construction applications. Testing, and reporting of test results are governed by such nationally recognized agencies as ASTM (American Society for Testing Materials). As an example, appended to this document is a copy of ASTM D3916 “Standard Test Method for Tensile Properties of Pultruded Glass-Fiber-Reinforced Plastic Rod”. For tensile testing, the “Tab Grip Adapters”, (FIG. 1, page 556) are constructed so as tendon (rod) failure does not occur at the grips as a result of the gripping action, but at the tendon in the area away from the grips. The area of contact between the grips and the tendon is of such a value as to allow transfer of the full ultimate load to the tendon uniformly. This action gives the true ultimate tensile strength of the tendon (rod) itself. The Universal Testing Machine, noted in the ASTM document generates the tensile force. The testing grips cannot practically operate independently of Universal Testing Machine. As a concurrence to the test results, the ultimate tensile strength may be calculated using tendon material component strengths. Tests, like that lastly noted are to verify that manufacturing processes produce materials to known values.
For practical applications, such as those aforementioned in construction, the gripping apparatus must be of a manageable size, have a method to assure the jaws (wedges) are set on to and engage the tendon, have a method to properly position the apparatus, and be re-useable many times without detailed maintenance, and in addition the apparatus must have the capability to be applied swiftly. As an example of manageable size, to use the testing grips in such applications already noted, for a 0.500″ diameter tendon (see ASTM D3916, “TABLE 1”) the device would be at a minimum of 24 inches long. For the same diameter tendon (rod) the instant apparatus is 2.50 inches long while still incorporating the features as presented. The sacrifice to meet these parameters is that the tensile strength of the tendon is limited to the relative action of the gripping apparatus components, primarily the action of the jaws (wedges). Unlike that used in laboratory the configuration of current tendon gripping devices limits this ultimate tendon tensile strength at failure is attributable to the nose of the jaws (wedges) biting into the tendon with continuing vigor until tendon (rod) tensile occurs. This is especially true when tendons comprised of fiberglass materials are used. The instant apparatus better transfers load to the tendon via unique interaction of the jaws (wedges) to the barrel and by the unique action of the jaws (wedges) to tendon (rod) engagement.
There are a number of parameters that govern load transfer from the jaws to the tendon. As illustrated through the ASTM testing procedure above the length, and subsequent area of engagement can be the main governing parameter. As noted for practical applications length and thus overall size of the apparatus is a strong consideration. As noted for the preferred embodiment the action of the jaws biting into the tendon limits the load capacity of the apparatus and tendon. The current invention incorporates novel methods to increase this load capacity while maintaining a manageable apparatus size. Firstly, the angle of incidence, or incident angle, between the tapered jaws and the internally tapered housing (barrel) is such that the under loading the rear portion, the large, butt end of the jaws are forced to more engage the tendon prior to the nose biting into the tendon. Secondly the instant apparatus incorporates a relieved, un-threaded portion at the internally threaded nose of the jaws. This last innovation, combined with the incident angle, greatly enhance the load bearing performance of the apparatus at a reduced length and thus tendon jaw contact area. These novel innovations will become apparent as this application continues.
Mentioned above is swift application of the apparatus. In addition to the necessity for swift application, the necessity for this method of application to be sturdy is paramount in the construction art to which it is envisioned that the device will be used primarily. Swift application for the instant is accomplished via the use of a speed thread having less threads per unit length (TPI—Threads Per Inch) then would a common machine nut. Standards for a common machine one inch diameter nut are 8-14 TPI. For the instant device with one inch threaded components the threads are at 5 TPI. With fewer TPI there is more threaded material available for load bearing and preclude possible damage. These innovations will become apparent as this application continues.
2. Discussion of Related Art
The art of tendon gripping devices is generally cognizant of gripping devices specifically designed for use with threaded tendons. Camming mechanisms used to secure tendon within a gripping device are also known. Representative prior art in the field of tendon gripping devices is shown below.
U.S. Pat. No. 5,154,558 discloses a smooth rod gripping device used in a blind anchoring situation.
U.S. Pat. No. 5,594,977 teaches the use of a smooth rod gripping device whereby the jaws are captured.
U.S. Pat. No. 4,192,481 discloses grippers that are specifically designed for use with threaded rod, and not a smooth rod. U.S. Pat. No. 2,614,801 discloses a wire holding and pre-stressing device.
U.S. Pat. No. 3,910,546 teaches a she-bolt type gripper device for a concrete wall formed tie rod. U.S. Pat. No. 3,965,542 is similar to preceding reference, and further adds a latch mechanism.
U.S. Pat. No. 1,634,422 discloses a rod clamp which operates by camming the tabs of opposing grip members within spiraled slots.
U.S. Pat. Nos. 2,075,239 and 2,171,120 both teach variations of a tie mechanism
U.S. Pat. No. 2,699,589 discloses a smooth rod clamping device. U.S. Pat. Nos. 2,896,496 and 3,117,485 teach the use of a spring within a shaft clamping mechanism. U.S. Pat. Nos. 4,192,215, 4,363,462 and 6,565,288 are additionally cited as of interest.
The need for an improved smooth and deformed or multiple stranded tendons still exists.
In the case of concrete formwork, including the use of multiple stranded tendons used as formties, including formwork used to construction the soil or rock embankment stabilization retaining wall structure, two opposing are erected to form a channel into which concrete is placed, they must be held together until the concrete sets. A smooth, or deformed, or multiple stranded tendon is passed through the formwork which is positioned on the outwardly facing surfaces of the structure to be constructed. The formwork, through which the tendon passes, serves as a base or platform for a tendon gripping device. A problem typical of smooth or deformed or multiple stranded tendons is that slippage occurs, allowing the walls to expand. Various presetting techniques, such as pounding a wedge shaped object between the gripping device and the formwork, have been utilized in attempts to minimize this slippage. The existence of springs in many gripping devices contributes to this slippage.